Current standards limit granulocyte storage to 24 hours. Since granulocyte colony-stimulating factor (G-CSF) inhibits granulocyte apoptosis, it may be possible to store G-CSF-mobilized granulocytes for longer periods while maintaining cell viability and function. However, G-CSF mobilization increases the yield of granulocytes several-fold and the resulting higher cell concentrations may diminish viability during storage.Granulocyte donors were given dexamethasone (8 mg PO), G-CSF (5 'g/kg SQ) or both and on the next day granulocyte concentrates were collected using a CS3000 blood cell separator. Component cell counts and pH were measured over time. Significantly more granulocytes were collected when donors were given G-CSF or both G-CSF and dexamethasone compared with dexamethasone alone. Storage had little effect on WBC count, but pH dropped significantly with time in all three types of granulocyte concentrates. Granulocytes mobilized with G-CSF plus dexamethasone were acidic immediately after the collection and pH was below 6.0 after 24 hours.Because acidic conditions are detrimental to cell survival, four granulocyte concentrates (two dexamethasone, one G-CSF and one G-CSF plus dexamethasone) underwent serial 2-fold dilution in autologous plasma prior to storage. pH was not maintained until the concentrates were diluted 8- and 16-fold. To optimize storage pH, mobilized granulocyte concentrates require an 8- to 16-fold dilution, which is operationally impractical. Studies that will evaluate the ability of clinical grade granulocyte preservative solutions to maintain pH during storage will begin soon. Additional measurements of viability and function will be used to evaluate such solutions.